In my late 20s (over 20 years ago), I picked up a rental violin and took it with me to the renaissance fair in Novato, California, and basically taught myself to play just by doing it on stage 7-8 times a day, twice a week, 8 weekends a year for many years. As you might imagine, I learned every single bad habit you can think of. One of them was how I do vibrato. In my early teens I taught myself blues guitar too, and played professionally for many years. You do vibrato in rock guitar by lightly bending the string. And that’s what I taught myself to do with the fiddle. Now in my early 50s, I’m trying to teach myself how to do it right…but since I have abysmal left hand positioning, getting proper vibrato is a major challenge.